syntax error – query useronline

Hi can someone explain this error to me ?

1064 – You have an error in your SQL syntax near ‘$sql[] = “CREATE TABLE $wpdb->useronline (“.
” timestamp int(15) NOT NULL d’ at line 1

THis is the code i got for useronline

// Create Useronline Table
$sql[] = “CREATE TABLE $wpdb->useronline (“.
” timestamp int(15) NOT NULL default ‘0’,”.
” username varchar(50) NOT NULL default ”,”.
” ip varchar(40) NOT NULL default ”,”.
” location varchar(255) NOT NULL default ”,”.
” url varchar(255) NOT NULL default ”,”.
” PRIMARY KEY (timestamp),”.
” KEY username (username),”.
” KEY ip (ip),”.
” KEY file (location))”;

I’m obviously doing something wrong here but what?

Thanks advance for your help.